feat(frontend): idempotency keys, request validation, Lighthouse budget, Statistics (#1340 #1341 #1349 #1351) - #1463

Description

Four issues from the PredictIQ frontend rebuild backlog, all in the frontend package.

• [API Client] Implement idempotency-key generation for mutating requests #1340 - src/lib/api/idempotency.ts generates a UUID Idempotency-Key per logical submission, resolved once outside the retry loop so automatic retries reuse it and each new submission gets a fresh one. placeBet / newsletterSubscribe opt in.
• [API Client] Validate client requests against generated schema types at the boundary #1341 - src/lib/api/requestSchemas.ts holds .loose() Zod schemas mirroring the generated OpenAPI request bodies (compile-time __contract ties them to components['schemas']). request() runs safeParse before fetch and throws ApiError(..., 'CLIENT_VALIDATION_ERROR') locally instead of letting the server 400.
• [Landing Page] Add Lighthouse performance budget for the landing page #1349 - Core Web Vitals budget (LCP 2500 ms / TBT 300 ms / CLS 0.1) in performance/config/thresholds.json; new pure evaluateMetricBudgets() in scripts/lighthouse-audit.js; the script now audits the landing route explicitly, writes lighthouse-latest.json, and exits non-zero when any category is below threshold or any metric is over budget.
• [Statistics Dashboard] Recreate Statistics component consuming GET /api/v1/statistics #1351 - src/components/Statistics.tsx recreated on the shared useAsync (status/retry) contract; data-driven tiles for Total Markets / Total Volume / Active Markets / Resolved Markets, every value coerced through toNumber() (number, decimal string, or absent -> 0, never blank). src/app/statistics/page.tsx minimally adapted to the same contract.

Full per-issue write-up: frontend/docs/pr/chideraisiguzor-1340-1341-1349-1351.md.

Type of Change

• Bug fix
• New feature
• Refactor / code cleanup
• Documentation update
• CI / tooling change
• Breaking change

Testing Done

From frontend/:

./node_modules/.bin/jest src/components/__tests__/Statistics.test.tsx \
  src/lib/api/__tests__/idempotency.test.ts \
  src/lib/api/__tests__/request-validation.test.ts \
  src/lib/api/__tests__/client.test.ts \
  scripts/__tests__/lighthouse-audit.test.js
# Test Suites: 5 passed, Tests: 106 passed

New suites: idempotency.test.ts (key reused across a 429-then-200 retry, stateful deduping-backend mock), request-validation.test.ts (malformed body rejected locally with fetch never called), lighthouse-audit.test.js (LCP/CLS over budget fails, missing metric fails), and the rewritten Statistics.test.tsx (real mocked /api/v1/statistics payload + an all-zero response asserting every tile renders 0/$0).

Pre-existing breakage, not touched: main is mid-migration to the new useAsync contract - tsc --noEmit reports ~351 errors, mostly other consumers still on the old { loading, execute } API. Only the two files named in #1351 are fixed here (count drops 360 -> 351, no new errors in touched files). LandingPage.accessibility.test.tsx also has 6 unrelated failing assertions. Left for their own issues per the contribution guidance.

Bundle Size

No dependency added (Zod is already a direct dependency). lighthouse / chrome-launcher are now lazy requires, so they no longer load unless the audit script actually runs. Runtime component/client changes are small; not separately measured.

Checklist

• Tests pass locally
• Documentation updated (if applicable)
• No breaking changes, or breaking changes are documented above
• If you added or changed an API endpoint, regenerated the OpenAPI spec - N/A, no API change
• If you changed system architecture - N/A
• Bundle size checked (if frontend changes) - see note above

Related Issues

Closes #1340
Closes #1341
Closes #1349
Closes #1351